"But that's just a fancy way of putting a link in the mail, more or less. Perhaps a neat integration of your email client with your cloud-storage."

Putting links in emails is only half the problem. I can send my parents files by uploading the files to my web server and emailing them a link. But they cannot send me files this way and neither can most people. Integrating email and P2P in a standard way would be a complete & efficient solution that everyone can use without much difficulty.

"problems and opportunities for webmail.
It would need to be an out-of-band delivery unless you wanted to redesign the SMTP protocol (which isn't going to happen)."

Oh I know that, even if it could be extended, I don't have any confidence that changes to SMTP would be widespread enough to make the plan viable. The only way such a system could work is on a new email platform where all clients supported these features from the get-go without worrying about legacy compatibility.